Film Bulk Acoustic Resonators—commonly known as FBARs—are used in a variety of applications, for example as radio frequency (RF) filters in wireless communication systems. In a typical membrane-type FBAR, the membrane is made up of a piezoelectric material sandwiched between a pair of electrodes. A voltage applied across the pair of electrodes causes the piezoelectric material to vibrate. The frequency of the first fundamental mode of a FBAR is determined by the thickness of the resonator's film stack, which is equal to the corresponding half-wavelength. RF FBAR filter is generally consisting two set frequencies of FBAR resonators.
During operation, how well a particular FBAR performs its function is related to the quality of the materials that make up the FBAR. In membrane-type FBARs, resonant properties are sensitive to the quality of the piezoelectric material sandwiched between the pair of electrodes. Currently, FBARs are manufactured by first depositing, patterning and etching a bottom electrode, then depositing the piezoelectric material on the already-etched bottom electrode and patterning and etching it, and finally depositing, patterning and etching of the top electrode.
Among other shortcomings, the current process for making FBARs significantly degrades the quality of the piezoelectric layer in the resonator. Patterning and etching the bottom electrode causes its surface to be irregular and rough. Depositing the piezoelectric material on such a rough surface distorts the orientation of crystals in the piezoelectric material, which can significantly degrade its quality. Moreover, the patterning and etching of the bottom electrode, which expose its surface to air, cause oxides and other substances to build up on its surface before the piezoelectric material is deposited. These oxides and other substances can also affect the quality of the piezoelectric layer.